Uninterruptible power supply (UPS) is an emergent power supply device connected between a power source and a load, in which the power source can be a commercial power supply or a regulated AC power. The main function of uninterruptible power supply is that the internal rechargeable battery of the uninterruptible power supply is configured to store electric energy when the power source is available for supplying power and release the stored energy to a load when the power source is unavailable for supplying power, in order to ensure the normal operation of the load.
In order to protect important electronic device efficiently and safely, UPS has been widely employed to ensure the normal operation of a variety of electronic devices. However, the on-line parallel-connected UPS is by far the most suitable choice for providing emergent power for electronic devices.
Referring to FIG. 1, a conventional parallel-connected uninterruptible power supply system is shown. As shown in FIG. 1, the parallel-connected uninterruptible power supply system 10 includes a power source 11, a first uninterruptible power supply 12, a second uninterruptible power supply 13, and a distribution box 14. The distribution box 14 includes a switch circuit 141 made up of switch elements 142 and 143.
When the switch element 142 within the switch circuit 141 of the distribution box 14 is ON, the first uninterruptible power supply 12 and the second uninterruptible power supply 13 are configured to receive an input AC power from the power source 11 through the switch element 142, respectively. The input AC power is rectified and filtered by the first uninterruptible power supply 12 and the second uninterruptible power supply 13, and thereby outputting energy. When the switch element 143 within the switch circuit 141 of the distribution box 14 is switched to be connected with the output terminals of the first uninterruptible power supply 12 and the second uninterruptible power supply 13, the energy outputted from the first uninterruptible power supply 12 and the energy outputted from the second uninterruptible power supply 13 is provided to power a load 15 in response to the load's demands.
Because both of the first uninterruptible power supply 12 and the second uninterruptible power supply 13 have the same circuit structure, it is intended to illustrate the circuit configuration and operating principle of the uninterruptible power supply by taking the first uninterruptible power supply 12 as an example. Referring to FIG. 1, the first uninterruptible power supply 12 principally includes an AC/DC converter 121, a charger circuit 122, a battery 123, a DC/DC converter 124, and a DC/AC converter 125.
The AC/DC converter 121 is configured to receive the input AC power from the power source 11 and convert the input AC power into a DC power. The charger circuit 122 is connected with the AC/DC converter 121 for receiving the DC power from the AC/DC converter 121 and converting the DC power into a DC voltage tailored to charge the battery 123.
When the power source is available for supplying power, the input AC power provided by the power source 11 is converted into a DC voltage by the AC/DC converter 121. The DC voltage outputted from the AC/DC converter 121 is converted into a DC voltage tailored to charge the battery 123 by the charger circuit 122. In the meantime, the DC voltage outputted from the AC/DC converter 121 is converted into an output AC voltage by the DC/AC converter 125, and the output AC voltage is outputted to the load 15 through the switch element 143 within the switch circuit 141.
When the power source is unavailable for supplying power, the battery 123 outputs a DC voltage which is then boosted by the DC/DC converter 124. The boosted DC voltage is delivered to the DC/AC converter 125 and converted into an output AC voltage by the DC/AC converter 125. The output AC voltage is delivered to the load 15 through the switch element 143 within the switch circuit 141.
The first uninterruptible power supply 12 and the second uninterruptible power supply 13 are connected in parallel with each other. Hence, when one of the uninterruptible power supplies is malfunctioned, the switch element must be OFF to break the connection between the power source 11 and the uninterruptible power supplies, and the switch element 143 must be switched to a bypass route so that the AC power provided by the power source 11 can be outputted to the load 15. In this manner, the wiring of the malfunctioned uninterruptible power supply is isolated from power and the malfunctioned uninterruptible power supply can be securely removed from the system for repair. Although the aforementioned parallel-connected uninterruptible power supply system can allow the malfunctioned uninterruptible power supply to be replaced with safety, the load 15 is directly powered by the output of the bypass route under this condition. In this case, the load 15 will be no longer protected by the uninterruptible power supply. If the power source is unavailable for supplying power, the load 15 can not operate normally.
Referring to FIG. 2, the structure of another conventional parallel-connected uninterruptible power supply system is shown. As shown in FIG. 2, the parallel-connected uninterruptible power supply system 10 also includes a power source 11, a first uninterruptible power supply 12, a second uninterruptible power supply 13, and a distribution box 14, wherein a plurality of breakers 144 are individually placed between the distribution box 14, the first uninterruptible power supply 12 and the second uninterruptible power supply 13.
The advantage of the above-mentioned uninterruptible power supply system is that when one of the uninterruptible power supplies is malfunctioned, the other one can provide power to the load 15. Under this condition, however, the breaker 144 connected to the malfunctioned uninterruptible power supply has to be closed first in order to isolate the wiring of the malfunctioned uninterruptible power supply from the power, so that the malfunctioned uninterruptible power supply can be removed from the system for repair.
The disadvantage of the above-mentioned uninterruptible power supply system is that a plurality of breakers 144 must be used, and the breakers 144 must be able to cut off all of the power-conducting terminals. For example, if the inputs of the uninterruptible power supplies 12 and 13 include a hot line and a neutral line, each of the breakers 144 must include two poles. Thus, the breakers 144 will be costly and space-consuming.
Therefore, it is necessary to develop a parallel-connected uninterruptible power supply system that can obviate the above-mentioned drawbacks.